A radar system of this kind is typically used in connection with an adaptive cruise control (ACC), which automatically adjusts the velocity of the host vehicle to the velocity of a preceding vehicle, to ensure that this preceding vehicle is followed at an appropriate collision-avoidance distance.
In an FMCW (frequency modulated continuous wave) radar commonly used for these purposes, the frequency of the transmitted radar signal is periodically modulated using different ramp slopes, and the radar signal reflected off of one or a plurality of objects is mixed with the transmitted signal, so that an intermediate frequency signal is obtained whose frequency corresponds to the frequency differential between the transmitted and the received signal.
A spectrum of the intermediate frequency signal is recorded in each measuring cycle through the use of an appropriate algorithm, such as the Fast Fourier Transform (FFT). In this spectrum, each located object is manifested as a peak at a specific frequency. The frequency at the peak location is dependent, on the one hand, on the ramp slope and on the object distance and, on the other hand, on the relative velocity of the object.
By comparing the peaks belonging to the same object measured at two different ramp slopes, the distance- and velocity-dependent frequency components are able to be separated, so that measurement data on the distance and the relative velocity of the object are obtained. The assumption in this context is that, ideally, the relative velocity remains virtually unchanged for the duration of the two ramps.
In the case of a plurality of objects, it is possible to eliminate any ambiguities that arise when assigning the peaks to the objects by evaluating at least one additional frequency ramp.
Since this evaluation procedure entails considerable computational outlay, a certain minimum duration is required for one single measuring cycle, so that the measurement data obtained are limited in terms of accuracy and temporal resolution.
In addition, so-called precrash systems are known, which are used for activating passive, reversible safety systems of the vehicle, such as reversible airbags, seat-belt tensioners and the like, in sufficient time, before an imminent impact when, on the basis of the radar data, the collision device recognizes a situation in which a collision can no longer be averted. Moreover, precrash systems are known which are used, inter alia, for determining an optimal firing point, for example, for a pyrotechnically actuated airbag or seat-belt tensioner. This requires a most accurate possible knowledge of the expected time of collision, as well as of the relative velocity at the time of collision, thus of the impact velocity.